Add the remote, call it "upstream":
git remote add upstream https://github.com/whoever/whatever.git
Fetch all the branches of that remote into remote-tracking branches, such as upstream/master:
git fetch upstream
Make sure that you're on your master branch:
git checkout master
Rewrite your master branch so that any commits of yours that aren't already in upstream/master are replayed on top of that other branch:
git rebase upstream/master
Push
git push -f origin master
Check out the branch you want to merge into
git checkout <feature-branch>Merge your (now updated) master branch into your feature branch to update it with the latest changes from your team.
git merge masterThis only updates your local feature branch. To update it on GitHub, push your changes.
$ git push origin <feature-branch>